The Ministry of Construction has reconsidered the fire-resistant design of building due to a significant increase in the height of buildings and advances in architectural design technique, etc. and the "New Fire-Resistant Design Law" was enacted in March, 1987. In the new Law, the limitation under the old Law that fireproofing should be provided so that the temperature of steel products during a fire is kept below 350.degree. C. has been removed, and it has become possible to determine a suitable fireproofing method depending upon a balance between the high-temperature strength of steel products and the actual load of building. Specifically, when the design high-temperature strength at 600.degree. C. can be ensured, the fireproofing can be reduced accordingly.
In order to cope with this trend, Japanese Unexamined Patent Publication (Kokai) No. 2-77523 proposes low yield ratio steels and steel products having an excellent fire resistance for use in buildings and process for producing the same. The subject matter of this prior application resides in that a high-temperature strength is improved by adding Mo and Nb in such an amount that the yield point at 600.degree. C. is 70% or more of the yield point at room temperature. The design high-temperature strength of the steel product has been set to 600.degree. C. based on the finding that this is most profitable in view of the balance between a increase in the steel production cost due to alloying elements and the cost of executing the fireproofing.
In the Al deoxidation of the steel in the prior art, Al has been added in an early stage of the production of a steel by the melt process, to effect deoxidation and floatation separation of the resultant Al.sub.2 O.sub.3, thereby purifying the molten steel. In other words, the subject matter was how to lower the oxygen concentration of the molten steel and to reduce the oxide as the product of the primary deoxidation.
The concept of the present invention is different from that of the above-described prior art. Specifically, the present invention is characterized in that a fine compound oxide useful as an intragranular ferrite transformation nucleus is precipitated and utilized by regulating the deoxidation process.
The present inventors have applied the steel produced by the above-described prior art technique to materials for shape steels, particularly an H-shape steel strictly restricted by roll shaping due to a complicated shape and, as a result, have found that the difference in the roll finishing temperature, reduction ratio and cooling rate between sites of a web, a flange and a fillet causes the structure to become remarkably different from site to site, so that the strength at room temperature, strength at a high temperature, ductility and toughness vary and some sites do not satisfy the JISG3106 requirements for rolled steels for welded structures.
In order to solve the above-described problem, it is necessary to attain a refinement of the microstructure through the device of steel making and rolling processes and provide a process for producing a controlled rolled shape steel having excellent material properties, fire resistance and toughness at a low cost with high profitability.